A typical ring type network of the prior art is shown in FIG. 1. For purposes of simplicity, it is assumed that all data propagates clockwise through the network. The network of FIG. 1 comprises ten exemplary Network Interface Units (NIU) 101-110. Communications from a first NIU to a second NIU is accomplished by passing data packets around the network from one NIU to the next. Each NIU reads the address in an arriving data packet. If the address matches the NIU's address, the data packet is processed by that NIU, and forwarded to the associated user equipment. Otherwise, the data packet is simply passed to the next NIU in the ring. This process continues until the data packet reaches the NIU for which it is destined. For example, a data packet to be transmitted from NIU 101 to NIU 104 is passed from NIU 101-102, from NIU 102 to 103, and from NIU 103 to 104.
Each packet transmitted through the ring network of FIG. 1 must be processed by each of the NIUs disposed between the transmitting and receiving NIUs. Therefore, packets are delayed substantially.
Recently, chordal rings have been proposed in order to help eliminate some of the delay problems posed by the network of FIG. 1. See for example, the article "Analysis of Chordal Ring Network", by B. W. Arden et al. in IEEE Transactions on Computers, Vol. C-30, No. 4, Apr. 1981, pp. 291-295, which analyses the reduction in delay achieved by these chordal ring networks. FIG. 2 shows a typical chordal ring network. The connections among the NIUs of FIG. 2 include all those of FIG. 1. Additionally, in FIG. 2. each NIU is arranged to communicate to the NIU disposed two NIUs away on the network. For purposes of explanation herein, the chordal length of the network is defined as the number of NIUs between the beginning and end of the chord, with the beginning NIU not counted. For example, the chordal length in FIG. 2 would be two.
The chordal ring network of FIG. 2 has much better delay performance than the ring network of FIG. 1. For example, consider the transmission of a data packet in the network of FIG. 2 from NIU 201 to NIU 206. The data packet could be transmitted from NIU 201 to NIU 203, from NIU 203 to NIU 205, and from NIU 205 to NIU 206. Note that only two intermediate NIUs must process the data packet, rather than four, as would be the case for the ring network of FIG. 1.
FIG. 3 shows a slightly more complex chordal ring network. In the network of FIG. 3, 16 NIUs are disposed along a communications medium. Each NIU transmits to the NIU immediately subsequent to it on the network. Additionally, each NIU is arranged to transmit to the NIUs which are disposed two NIUs subsequent to, and four NIUs subsequent to, the NIU in question. For example, NIU 309 transmits to NIU 310, NIU 311, and NIU 313. To analogize to FIG. 2, the network of FIG. 3 has two separate chordal lengths, two and four.
The networks of FIGS. 2 and 3 are a compromise between a fully connected network, where each NIU is connected to every other NIU, and the ring network of FIG. 1. The networks of FIGS. 2 and 3 reduce delay at the cost of additional connections.
Both the networks of FIGS. 2 and 3, as well as other chordal ring networks, suffer from a very practical limitation with regard to their implementation. Specifically, the medium is typically implemented as a bundle of wires or optical waveguides. Considering cross section 317 of FIG. 3, it can be seen that at least seven communications channels are required to implement the chordal ring network. It can also be seen from the network of FIG. 3 that each NIU is inserted serially into a selected three of these seven channels, while the remainder simply propagate by the NIU untouched. Thus, in order to implement the network of FIG. 3, the installer must determine which wires to connect to each NIU from a bundle of wires. This problem becomes quite substantial when the network gets very large; i.e. hundreds or thousands of NIUs, and the number of chords is made greater.